spheery clime;" but if we are to fly it will be by humbly following Nature and learning what she has to teach. There is no study so certainly rewarded as this pursuit of Nature. He who courts her honestly, patiently, earnestly, she will by no means send empty away. As an old Roman said, it is well when we can mix the pleasant with the useful and the good; and, in the study of Nature, this must be ever so; for her ways, like Wisdom's, are "ways of pleasantness, and all her paths are peace."

A LECTURE, Delivered in the Hulme Town Hall, Manchester, on Wednesday, December 3rd, 1873.

By PROFESSOR CROOM ROBERTSON, M.A.

UPPOSE, by a wild stretch of imagination, some mechanism that will make a rod turn round one of its ends, quite slowly at first, but then faster and faster, till it will revolve any number of times in a second; which is, of course, perfectly imaginable, though you could not find such a rod or put together such a mechanism. Let the whirling go on in a dark room, and suppose a man there knowing nothing of the rod: how will he be affected by it? So long as it turns but a few times in the second, he will not be affected at all unless he is near enough to receive a blow on the skin. But as soon as it begins to spin from sixteen to twenty times a second, a deep growling note will break in upon him through his ear; and as the rate then grows swifter, the tone will go on becoming less and less grave, and soon more and more acute, till it will reach a pitch of shrillness hardly to be borne, when the speed has to be counted by tens of thousands. At length, about the stage of forty thousand revolutions a second, more or less, the shrillness will pass into stillness; silence will again reign as at the first, nor any more be broken. The rod might now plunge on in mad fury for a very long time without making any difference to the man; but let it suddenly come to whirl some million times a second, and then through intervening space faint rays of heat will begin to steal towards him, setting up a feeling of warmth in his skin; which again will grow more and more intense, as now through tens and hundreds and thousands of millions the rate of revolution is supposed to

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rise. Why not billions? The heat at first will be only so much the greater. But, lo! about the stage of four hundred billions there is more—a dim red light becomes visible in the gloom; and now, while the rate still mounts up, the heat in its turn dies away, till it vanishes as the sound vanished; but the red light will have passed for the eye into a yellow, a green, a blue, and, last of all, a violet. And to the violet, the revolutions being now about eight hundred billions a second, there will succeed darkness-night, as in the beginning. This darkness too, like the stillness, will never more be broken. Let the rod whirl on as it may, its doings cannot come within the ken of that man's senses.

This experimental fancy-rather apt to take the breath awayI quote from the German books where it is to be found, because it brings into line, in a striking way, the most of what physical science can tell us about the senses, and at the same time suggests a number of questions, which, though they go beyond physics to answer, are among those that we must try to deal with this evening. Physics, as you know, is the science treating of nature, or the world of matter; and it explains what it can of the changes or processes going on there by resolving them into motions, under some general laws that have been very certainly determined. Now a great part of all the changes in nature are in the sensible qualities of things, such as their colour, temperature, and the like; and for all the variety of these the physical inquirer seeks out an expression in terms of motion. That in the objects sound, colour, &c., are motions, however they may appear to particular senses, was long ago surmised; as, indeed, in the case of sound, which first began to be understood, the fact is often quite evident. Sonorous bodies like a bell or a drum or a musical string are plainly in motion, which may pass to other bodies, and in particular by one great body, the air, can be carried a long way. The motion in bodies when giving forth light or heat, and the medium-not air-which is the general bearer of that motion, have been much less easy to determine; but modern inquiry has practically mastered the difficulty, and the tremendous figures given in our fancied experiment are some of those assigned in all soberness for the number of vibrations per second in the all-pervading ether that go with simple sensations of heat and colour in us. There is no expression of the same definite kind for tastes and smells; the process there being of the chemical rather than of the mechanical sort. But a chemical action also is, in the last resort, intelligible to us only as a mode of motion; and thus we may say that all sensible qualities are resolved by physical

science into motions in the objects. In touch, which has not been mentioned, the action is mechanical of the most apparent kind.

Now, coming back to our rod, whose whirling is supposed to communicate to the air and ether in the room motions of like rate to those caused in fact by sounding, hot and shining bodies, we may remark two things strange. The first is that its motion had no effect on the man except at particular stages, and within a definite range at each. Putting always aside the case of actual contact as practically out of the question, we note the blank before the first deep groan burst forth, the tremendous blank when the last screech had gone out until heat began to steal in, and again the immeasurable tract lying beyond the limit where light passed into darkness. The second fact is that within a certain range the motion appeared differently as both heat and light. Why should one rate of the motion appear only as sound, another only as heat, and another only as light? Why should other rates among or outside of these not appear as anything at all? And why should one rate appear doubly as both heat and light? These are questions that do not concern the physical inquirer, whose work is done when he has got the sensible qualities into expressions admitting of definite measurement. But we must try to find an answer for them.

There can be no doubt in what direction we have first to look. The question is why bodies outside of us affect us in certain ways and not in others. Well, of course, that depends on our capacity of being affected. Our physical frame or body offers itself to be acted on by other bodies in motion, and the result in the first instance must depend upon what organs and what kind of organs it has for receiving the motion or stimulus. This it is the

business of a different man of science, the physiologist, to determine; and within the last generation or two-not earlier-a great deal has been done for the physiology of sensation, however much remains to be learned.

In regular sensation, as of a colour or sound, there is an invisible disturbance in some part or parts of the mass of the brain within the skull. This disturbance results from an ingoing wave or current of invisible motion along the white fibrous lines called nerves. This wave or current begins at the outer ends of the nerve-fibres, where they are in conjunction with various microscopic structures, partly nervous, partly other than nervous; and these structures are reached by the exciting stimulus (which we have seen to be some motion, visible or invisible, in external bodies), through the parts or openings on the surface of the human bodyeyes, ears, and the like-which are commonly called the organs of

the senses. It is a very complex process altogether, and for true sensation all the stages are of account; yet some are easily seen to be of greater importance than others. Least important is the part played by the external organs, for these are often injured without sensation being stopped. Most important is the action of the brain, without which there can be no conscious state at all. For the rest, let us carefully distinguish between the nerve-fibres going to the brain and their endings in the minute structures. Nerve-fibres, by themselves, are mere conductors which, like telegraph-wires, may carry indifferently in either direction, and, though in the actual nerves, which are compound bundles of fibres, they carry only one way, they will carry any sort of disturbance, whatever it be, that is strong enough to rouse them at all. Thus the optic nerve may be excited by any strong pressure, and is not excited when acted on directly by the proper stimulus of light, which happens to be a very weak one. In short, the fibrous lines of nerve seem not to determine the character of the sensations had through them, any more than a telegraph-wire determines the import of the message sent along it. But, if the mere nerves are practically alike in structure and function, most varied is the structure of their endings at the outer organs. The endings in ear, and eye, and skin are quite different; and, again, at different parts of the same organ-as between the middle and sides of the back of the eye, or between the finger-tips, and skin of the shoulders in the organ of touch-the variety of structure is very great. Note this second point, because we shall come back upon it later. It is the first point that concerns us now.

Besides the fibres, it should be observed that the nervous system includes another sort of matter, consisting of darker-coloured cells, extremely minute in size. These cells, wherever found-in little gatherings here and there, or compacted into a column at the heart of the spinal cord, or massed variously at the base of the brain, or packed away in the winding folds next to the skull-capare storehouses of pent-up energy, ready, upon the least excitement, to burst forth as invisible motion along fibrous lines laid from them. The fibres are in substance much less unstable, and, besides, both singly and as done up in bundles, or again in the sets of bundles which are called nerves, are protected by sheaths along their whole length, the ends only being left exposed. Now, as the brain buried away in the skull is, in the regular process of sensation, thrown into action only by the disturbance sent up along the nerve-fibres from their tiny ends thus unprotected, the stimulus applied here must either be very strong in itself, or, it